The tests expect a couple of global variables to exist, JS and Set.
CommonJS platforms encourage us to avoid global variables and so we need to
import the modules we need into each spec file. Add the following to the top
of example/spec/set_spec.js to load jstest and the Set class.

If you want to run the tests both on the server and in the browser, you’ll
either need to use a tool like
browserify to resolve the
require() statements, or add a little more boilerplate around the test so it
can find the variables it needs whether they are globals or CommonJS modules.

(If you use browserify, you’ll need to pass --noparse jstest on the
command line. jstest is a self-contained file that’s already gone through a
browserify-like build process, and you’ll get lots of errors unless you tell
browserify not to process it.)